1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly, to a system and method for performing a handover between networks with different communication modes.
2. Description of the Related Art
With the advance of modem society, mobile communication systems have made rapid technical progress and their services have been provided in various communication modes. In order to provide mobile communication services in various communication modes, a new mobile communication mode does not replace an existing one abruptly, but initially operates in a limited manner in a network used for demonstration purposes only (i.e., a local network including mobile communication system equipment for replacing an existing one to support the new mobile communication mode). While operating in this mode, when a user terminal moves out of the demonstration operation network while receiving a communication service in the new mobile communication mode, it can be continuously provided with the communication service only in the case when a handover can be performed between the network employing the new mobile communication mode and an existing mobile communication network employing the existing mobile communication mode. If a handover cannot be performed between the network employing the new mobile communication mode and the existing mobile communication network, the communication service being provided to the user terminal will be interrupted and will not be able to be provided.
A brief description will hereinafter be given of the structures of the network employing the new mobile communication mode and the network employing the existing mobile communication mode with reference to FIG. 1. FIG. 1 shows the construction of a general mobile communication system, wherein the new mobile communication mode is assumed to be a wide-band code division multiple access (referred to hereinafter as “WCDMA”) mode, which is a 3rd generation (3G) mobile communication mode. The existing mobile communication mode is assumed to be a code division multiple access (referred to hereinafter as “CDMA”) mode, which is a 2nd generation (2G) mobile communication mode.
FIG. 1 schematically shows the construction of the general mobile communication system.
With reference to FIG. 1, on the assumption that the new mobile communication mode is the WCDMA mode and the existing mobile communication mode is the CDMA mode, as stated above, the network employing the WCDMA mode (referred to hereinafter as “WCDMA network”), denoted by the reference numeral 100. Network 100 includes a user terminal (referred to hereinafter as “WCDMA user terminal”) 111 for receiving a communication service in the WCDMA mode, a node B 113 (i.e., a 3G base station), a radio network controller (RNC) 115, and a universal mobile telecommunications system (UMTS) mobile switching center (referred to hereinafter as “UMSC”) 117. The RNC 115 is connected with a core network (CN, not shown) to perform all processes associated with connections of the WCDMA user terminal 111. The RNC 115 also acts to allocate radio resources to the WCDMA user terminal 111 when it is connected with the node B 113. The node B 113 acts to allocate channels to the WCDMA user terminal 111 on an actual physical layer.
The network employing the CDMA mode (referred to hereinafter as “CDMA network”), is denoted by the reference numeral 150. Network 150 includes a user terminal (referred to hereinafter as “CDMA user terminal”) 151 for receiving a communication service in the CDMA mode, a base transceiver subsystem (BTS) 153, a base station controller (BSC) 155, and a mobile switching center (MSC) 157. The BTS 153, BSC 155 and MSC 157 perform the same functions as those of the node B 113, RNC 115 and UMSC 117, respectively, only with the exception that the service mode is the CDMA mode, not the WCDMA mode, and a detailed description thereof will thus be omitted.
In the mobile communication system with the construction described above, the WCDMA user terminal 111 may approach the service area of the CDMA network 150 while receiving communication services in the service area of the WCDMA network 100. In this case, a handover must be performed between the WCDMA network 100 and the CDMA network 150. However, the different communication modes of the WCDMA network 100 and CDMA network 150 make it impossible for the WCDMA user terminal 111 to hand over to the CDMA network 150, although it will attempt the handover.
Therefore, in order to support a handover from a WCDMA mode-based communication system (or a 3G mobile communication system), to a CDMA mode-based communication system (or a 2G mobile communication system), accurate consideration must be taken into account for parameters essential to the WCDMA mode-based communication system and parameters essential to the CDMA mode-based communication system. Further, in order to support the handover from the WCDMA mode-based communication system to the CDMA mode-based communication system, the currently defined air-interface standard cannot be subjected to any variation, which would make it impossible to perform the handover from the WCDMA mode-based communication system to the CDMA mode-based communication system.